A typical turbo fan gas turbine engine has a core engine which drives a fan at the front of the engine. The fan is part of a fan assembly comprising an array of fan blades mounted on a hub. The hub is a rotating component and is situated axially close to a stationary inner annulus defining the inner boundary of the gas flow path through the core engine. The interior of the inner annulus is typically under pressure, and it is necessary to provide a seal between the hub and the inner annulus to prevent leakage of pressurized air from the interior of the inner annulus.
To achieve this seal, the fan assembly includes a sealing component known as a fan rear seal which is secured to the hub and has an annular sealing surface which makes sealing contact with a complementary surface on, or fixed to, the inner annulus. Typically, fan rear seals are multi-piece machined forgings of aluminum or titanium alloy.
As well as providing a gas seal, fan rear seals also act as pistons, balancing load between the fan and the low pressure (LP) turbine that drives it. Fan rear seals are also required to support an axial force that gives a bearing end load, and also to protect the other components of the fan assembly from the high temperatures generated in the core engine, which could reduce the fatigue life of the fan components.
Fan rear seals made from alloy forgings are expensive and difficult to machine. The components are also heavy and are subjected to high stress and high vibration, which contributes to low fatigue lives. Damping mechanisms, such as damping rings, are often fitted to the fan rear seals in an attempt to reduce vibratory stresses, and this adds to the overall weight of the component. Aluminum alloys are relative inexpensive and easy to machine, but cannot withstand high temperatures and suffer from a low fatigue life and high thermal growth. Titanium alloys have superior physical properties, but are expensive and difficult to machine.